test: Clean up site_login.py a bit.
This has overlap with rginda's
http://codereview.chromium.org/1534001, but there's
one part that I prefer from my change: making all
of these wait_ functions throw exceptions when they
time out, which cuts down on a bunch of code that
would otherwise need to be duplicated across all of
our tests.
I also ran into a bunch of races in UI tests when I
reduced the polling timeout, so I've tried to fix
those too.
BUG=none
TEST=ran the tests, both logged-in and logged-out